The present invention relates to methods of processing hydrocarbon gases. In particular, it relates to methods of limiting the carbon dioxide content of liquids produced from natural gas liquids recovery processes.
Ethylene, ethane, propylene, propane and/or heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, usually in excess of 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases. The present invention is generally concerned with the recovery of ethylene, ethane, propylene, propane and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 92.5% methane, 4.0% ethane and other C2 components, 1.0% propane and other C3 components, 0.20% iso-butane, 0.20% normal butane, 0.10% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur-containing gases are also sometimes present. The historically cyclic fluctuations in the prices of both natural gas and its natural gas liquid (NGL) constituents have at times reduced the incremental value of ethane, ethylene, propane, propylene, and heavier components as liquid products. Competition for processing rights has forced plant operators to maximize the processing capacity and recovery efficiency of their existing gas processing plants. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, and refrigerated oil absorption. Additionally, cryogenic processes have become popular because of the availability of economical equipment that produces power while simultaneously expanding and extracting heat from the gas stream being processed. Depending upon the pressure of the gas source, the richness (ethane, ethylene, and heavier hydrocarbons content) of the gas, and the desired end products, each of these processes or a combination thereof may be employed. The cryogenic expansion process is now generally preferred for natural gas liquids recovery because it provides maximum simplicity with ease of start up, operating flexibility, good efficiency, safety, and good reliability. U.S. Pat. Nos. 4,157,904; 4,171,964; 4,185,978; 4,251,249; 4,278,457; 4,519,824; 4,617,039; 4,687,499; 4,689,063; 4,690,702; 4,854,955; 4,869,740; 4,889,545; 5,275,005; 5,555,748; 5,568,737; 5,771,712; 5,799,507; 5,881,569; 5,890,378; reissue 32141-60/110,502 U.S. Pat. Nos. 33,408; and U.S. Pat. No.5,983,664 describe relevant processes which are well known in the art. In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high-pressure liquids containing some of the desired C2 and heavier components. Depending on the richness of the gas and the amount of liquids formed, the high-pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquids results in further cooling of the stream. Under some conditions, pre-cooling the high pressure liquids prior to the expansion may be desirable in order to further lower the temperature resulting from the expansion. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer) column. In the column, the expanded cooled stream(s) is (are) distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired C2 components, C3 components, and heavier hydrocarbon components as bottom liquid product. If the feed gas is not totally condensed (typically it is not), at least a portion of the vapor remaining from the partial condensation can be passed through a work expansion machine or engine, or an expansion valve, to a lower pressure at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is essentially the same as the pressure at which the distillation column is operated. The combined vapor-liquid phases resulting from the expansion are supplied as a feed to the column. In recent years, the preferred processes for hydrocarbon separation involve feeding this expanded vapor-liquid stream at a mid-column feed point, with an upper absorber section providing additional rectification of the vapor phase. The source of the reflux stream for the upper rectification section is typically a portion of the above mentioned vapor remaining after partial condensation of the feed gas, but withdrawn prior to work expansion. An alternate source for the upper reflux stream may be provided by a recycled stream of residue gas supplied under pressure. Regardless of its source, this vapor stream is usually cooled to substantial condensation by heat exchange with other process streams, e.g., the cold demethanizer tower overhead. Some or all of the high-pressure liquid resulting from partial condensation of the feed gas may be combined with this vapor stream prior to cooling. The resulting substantially condensed stream is then expanded through an appropriate expansion device, such as an expansion valve, to the pressure at which the demethanizer is operated. During expansion, a portion of the liquid will usually vaporize, resulting in cooling of the total stream. The flash expanded stream is then supplied as top feed to the demethanizer. Alternatively, the cooled and expanded stream may be supplied to a separator to provide vapor and liquid streams, so that thereafter the vapor is combined with the demethanizer tower overhead and the liquid is supplied to the column as a top column feed. In liquid recovery facilities of the type described here, the bottom product leaving the demethanizer comprising primarily of C2 and heavier components along with carbon dioxide and methane components may be sent to subsequent fractionation towers. The first such fractionator is a deethanizer in which substantially all the C2, carbon dioxide and methane components are separated as a top product and the substantially all the C3 and heavier components are produced as a bottom productThe purpose of the overall plant is to produce residue gas leaving the process which contains substantially all of the methane in the feed gas with essentially none of the C2 components and heavier hydrocarbon components, an ethane liquid product leaving the deethanizer overhead which contains substantially all of the C2 components while meeting plant specifications for maximum permissible methane and carbon dioxide content, and a bottoms liquid stream leaving the deethanizer containing the C3 and heavier hydrocarbon components with essentially no ethane or more volatile components.
The present invention provides a means for providing a new plant or modifying an existing processing plant to achieve this separation at significantly lower capital cost by reducing the size of or eliminating the need for a product treating system for removal of carbon dioxide from the C2 stream.
In U.S. Pat. No. 6,182,469, the contents of which are incorporated herein by reference, a method of processing hydrocarbon gas is disclosed which increases carbon dioxide rejection in a cryogenic NGL gas recovery process. Essentially, a heat input (reboiling) is provided higher in the main distillation column (the demethanizer) which rejects more carbon dioxide into the residue gas. However, this solution requires a reconfiguration of the trays inside the demethanizer to retrofit an existing gas processing plant. In some cases, a new side heat exchanger may be required in a retrofit.
Therefore, there is a need in the art for alternative or improved methods of controlling carbon dioxide in a cryogenic gas processing process.
In a typical cryogenic expansion recovery process, the bottom stream from the demethanizer is typically sent for fractionation in a deethanizer where the ethane (C2), carbon dioxide and lighter components are separated from the propane and heavier (C3+) components. The overhead of the deethanizer is typically configured with a condenser which takes all of the C2, carbon dioxide and lighter components as a vapor from the deethanizer and completely condenses it to make a liquid product comprising primarily of C2 and smaller amounts of carbon dioxide and lighter components. The current invention does not condense all the vapour leaving the deethanizer overhead but a portion of the vapour is recycled back to an upstream point, typically directly into an upper portion of the demethanizer column. This recycle vapor stream is always richer in methane and carbon dioxide than the liquid ethane product stream, which allows the liquid ethane product stream to be lower in carbon dioxide and methane even though the liquid stream leaving the bottom of the demethanizer may have excessive amounts of methane and carbon dioxide. The amount of vapor recycle is directly controllable by raising or lowering the cooling to the overhead stream from the deethanizer thereby controlling the degree of condensing of the vapor stream leaving the deethanizer.
Therefore, in one aspect, the invention comprises an improvement to a cryogenic natural gas liquid extraction process wherein a gas stream comprising methane, carbon dioxide, C2 components and heavier hydrocarbons is first separated in a demethanizer into a volatile fraction comprising primarily methane and a relatively less volatile fraction comprising C2, carbon dioxide and heavier hydrocarbon components; and wherein the less volatile fraction is then separated in a deethanizer into a fraction comprising primarily C2 components and a fraction comprising heavier hydrocarbon components; wherein the improvement comprises the step of recycling a portion of the C2 fraction as a vapour to the demethanizer or alternatively to any point upstream of the demethanizer.
In another aspect, the invention comprises an improvement to a cryogenic natural gas liquid extraction process wherein a gas stream comprising methane, carbon dioxide, C2 components and heavier hydrocarbons is separated into a volatile fraction comprising primarily methane and a relatively less volatile fraction comprising C2 components and heavier hydrocarbon components, in which process: (a) said gas stream is treated in a processing facility containing heat exchange and expansion devices typical to the cryogenic liquid extraction industry and is fed in a number of split streams to a demethanizer in which substantially all of the methane and lighter gases leave as a top product and substantially all of the C2 and heavier components leave as a bottom product.
(b) the relatively less volatile fraction leaving the bottom of the demethanizer is fractionated in a deethanizer to separate the relatively less volatile fraction into a fraction comprising primarily of hydrocarbons heavier than C2 components and a C2 product stream which contains essentially all of the methane and carbon dioxide which leave the bottom of the demethanizer; (c) the vapour leaving the top of the deethanizer is partially condensed to form a liquid C2 product stream; wherein the improvement comprises the step of recycling all or a portion of the uncondensed vapour portion of the deethanizer overhead back to the demethanizer or to any point in the process upstream of the demethanizer.